Method and apparatus for deploying articles in deep waters

ABSTRACT

Equipment and method for lowering equipment ( 10 ) to the seabed from a vessel ( 12 ) using man made fibre rope ( 16 ) and a winch ( 20 ) employing wire rope ( 24 ), the fibre rope being paid out (or when lifting, drawn in) in sections by repeated operation of the winch ( 20 ) and wire rope ( 24 ). This is preferably done by holding the fibre rope ( 16 ) with holding means ( 32 ), while the wire rope ( 24 ) is detached for connection to a next section. The holding means ( 32 ) may be arranged to engage an attachment point ( 18 ) between adjacent rope sections but distinct from that engaged by the hoist platform ( 22 ).

The invention relates to methods and apparatuses for deploying articlesto great depth beneath the sea surface, for example to the seabed indeep waters.

Cranes and winches employing wire rope have been used to deploy loads tothe seabed in modest water depth for many years. Some of these cranesand winch systems are fitted with, or used in conjunction with, heavecompensators, which take-up and pay out the rope dynamically, tocompensate vertical motion (heave) of the ship, barge or other platformfrom which the rope is supported.

As water depth increases, the weight of wire needed to lower equipmentto the seabed increases until it becomes such a significant part of thetotal load that the method becomes impractical. Man made fibre rope canbe almost neutrally buoyant and have strength and elasticcharacteristics similar to wire rope and is therefore potentially asuitable replacement for wire. Man made fibre rope, however, has a poortolerance to the fatigue induced by bend cycling under load, and is thusunsuitable for use with current designs of heave compensator or withheave compensated drum winches. The same problem exists for winchsystems without heave compensation, although the bend cycling willtypically be less severe than in heave-compensated systems. Alternativesystems can also be envisaged which do not increase bend cycling for thepurpose of heave compensation, but bend cycling for the basiclifting/lowering operation is harder to avoid.

The invention aims to obtain the weight benefits of using fibre rope,while avoiding the need for bend cycling fibre rope under load, whendeploying loads from a vessel at sea.

Broadly stated the invention provides equipment and method for loweringequipment to the seabed from a vessel using man made fibre rope and awinch employing wire rope, the fibre rope being paid out (or whenlifting, drawn in) in sections by repeated operation of the winch andwire rope.

The fibre rope may be continuous and provided with eyes, stoppers orother attachment points at regular intervals.

The fibre rope may alternatively comprise discrete sections terminatedwith eyes or other attachment points, connected together to form therequired length. In this case, the connections between sections may bemade (or un-made) in the course of paying out (drawing in), or theentire length may be connected in advance and reeled during operation.

Each fibre rope section may be terminated by looping around a thimblecomprising a body with a peripheral channel for the rope, and twodistinct openings, one opening receiving a connection (directly orindirectly) to the next rope sections, the other receiving a connection(directly or indirectly) to the hoist mechanism. In a particularembodiment disclosed in detail below, the second opening receives a boltwhich fixes a stopper to the thimble, the stopper being temporarilyengaged by the hoist mechanism.

The stopper comprises a part-conical member formed in two halves heldtogether by the bolt. In other arrangements, it can be envisaged thatthe hoist mechanism engages the thimble directly.

The thimble having two distinct apertures and optionally a stopperpermanently attachable thereto is also an independent aspect of theinvention, for which novelty is claimed.

The fibre rope may be provided between adjacent rope sections withstoppers engaged and released by a hoist mechanism openable by remotecontrol to engage and release the fibre rope.

In a preferred embodiment, the wire rope is arranged to raise and lowera hoist mechanism, the hoist mechanism being operable by remote controlto allow the mechanism to pass freely up and down the rope, and then toengage the rope by means of said stopper.

The fibre rope may be held at a holding means, while the wire rope isdetached for connection to a next section. The holding means may bearranged to engage an attachment point between adjacent rope sectionsbut distinct from that engaged by the hoist platform. A pair of collarsor other stoppers is sufficient, spaced longitudinally to allowsufficient clearance between hoist mechanism and the holding means.

Preferably the wire rope winch is heave compensated, permittingoperation in a wider range of sea states and reducing strain on thecomponents of the apparatus.

The wire rope winch and holding means may both be heave-compensated,such that heave compensation can be maintained throughout the process oftransferring the load from one to the other. The holding means and wirerope winch may be heave compensated in parallel by a common heavecompensator, for example comprising a hydraulic ram.

The winch and wire rope may be arranged in a double fall arrangement,with the end of the wire rope fixed on board the vessel while a runningblock is lowered and raised, alternately connected to and disconnectedfrom the fibre rope.

The invention further provides an apparatus comprising a rope store,hoist means and holding means adapted for deployment from a sea-goingvessel for implementing a method of raising or lowering according to theinvention as set forth above.

The invention further provides a fibre rope assembly comprising pluralrope sections and load-bearing stoppers connected between the sections,the rope assembly being adapted for use with a method according to theinvention as set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows the general arrangement of a lifting apparatus according toone embodiment of the present invention;

FIG. 2 shows in more detail a hoist system within the apparatus of FIG.1;

FIG. 3 shows in more detail the holding means including a hang-offplatform in the apparatus of FIG. 1;

FIG. 4 shows a fibre rope system within the apparatus of FIG. 1,including enlarged detail of a joint between two rope sections;

FIGS. 5, 6, 7, 8 and 9 illustrates successive stages in the operation ofthe apparatus, lowering a load to the seabed by a method according tothe present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

General Arrangement

FIG. 1 shows the general arrangement of a heave compensated liftingapparatus lowering a load 10 over the side of a vessel (not shown infull) in deep water. The load in this case is shown as a simple clumpweight, but of course could be any module or tool which requires to bedeployed to a great depth beneath the sea surface. Depths well in excessof 1000 m are routinely encountered in offshore developments, althoughthe invention is not limited to any particular range of depth. A portionof the deck of the vessel is indicated schematically at 12, while theside wall of the vessel is indicated at 14. This could equally be theside wall of a moonpool, depending for example on the size of the loadto be deployed.

Load 10 is suspended chiefly on a man-made fibre rope 16, made insegments with joints 18 along its length. A drum winch 20 mounted ondeck 12 stores sufficient rope 16 for the depth of operation. The lengthof each segment may be 50 m, 100 m, or 300 m for example.

A hoist platform 22 is suspended over the side of the vessel by adouble-drop steel wire 24 a hoist winch 26 provided on deck 12 storesthe wire 24. Rope 16 passes through hoist jaws 28 mounted on hoistplatform 22, and engages part of a joint 18, such that the weight of theload 10 is carried by hoist platform 22 and will stop the tension causedby the weight of the load is thus not experienced by portions of rope 16above the level of platform 22.

A hang-off platform 28 is also suspended by wires 30 a and 30 b hang-offplatform 28 carries a latch 32 which can be opened and closed to engageanother portion of the joint 18 in rope 16, which passes through anaperture in hang-off platform 28. Hang-off platform 28 acts as a holdingmeans and is arranged to stay at a relatively fixed height in relationto this sea surface, while hoist platform 22, by operation of hoistsystem winch 26, travels from a level just below hang-off platform 28 toa depth at least corresponding to the length of one segment in fibrerope 16.

Finally in this embodiment, FIG. 1 shows a heave compensator 34,comprising a number of sheaves (pulley wheels) mounted on a cross head36 which is connected to the deck by a hydraulic ram 38. Rope 16 andwires 24 and 30 all pass over respective sheaves mounted on cross head36. In operation, ram 38 is controlled automatically in response toheave (vertical motion) of the vessel, so as to isolate high-offplatform 28 and hoist platform 22, and hence load 10, from this verticalmotion. The principles of heave compensation are well-known, andappropriate systems are available from a number of manufacturers. Thedetail of the heave compensation system will not be described further.Moreover, it will be appreciated that heave compensator 34 can beomitted if it is not necessary in a given application.

The arrangement shown in FIG. 1 can be adapted to function without heavecompensation, simply by eliminating ram 38 and envisaging the sheavescarried by cross head 36 being mounted at a fixed height above deck 12.

The various components of the lifting apparatus shown in FIG. 1 will bedescribed in more detail below. It should be appreciated that thedetailed arrangement is only one possible example.

Hoist System

FIG. 12 shows in more detail the hoist system platform and the manner inwhich it is suspended by wire 24. Heave compensator 34 is shown inbroken outline, to avoid obscuring relevant detail. In this drawing,platform 22 is shown as a simple steel plate, which may of course bereinforced according to its load-bearing function. Aperture 40 can beenseen, through which rope 16 and joint 18 can pass freely (the apertureis shown smaller than life in the figure to improve its visibility).Jaws 27 a and b are provided each with half-conical bearing surfaces 42a and 42 b for engaging part of a joint 18 in rope 16. The detailed formof the joints in the rope will be described below in relation to FIG. 4.Jaws 27 a and 27 b are open and closed by means of hydraulic rams 44 aand 44 b. An inset detail 46 shows the jaws in the closed position andengaging a joint 18 of rope 16.

As mentioned already, hoist platform 22 is supported in operation bywire rope 24. Three sheaves 48, 50 and 52 are provided on hoist platform22, for guiding the wire 24 from two points of suspension at either endof the platform around the area of the aperture 40. By this arrangement,vertical load on the jaws 27 a, 27 b acts substantially on the linebetween the two portions of wire 24.

Also associated with the hoist system platform and wire rope 24 arehoist winch 20 already mentioned, a strong point 53 on deck 12, firstand second sheaves 54 and 56 mounted on the heave compensator cross head36 and a fixed sheave 58 mounted on the deck. The mounting and bearingsof these sheaves are omitted for clarity. The functioning of the hoistwill be described later.

Holding Means/Hang-off Platform

FIG. 3 shows in more detail the hang-off platform 28 and associatedcomponents. Again this comprises essentially a steel plate with anaperture 60 for passage of fibre rope 16 and joints 18. Latch 32comprises first and second latch pieces 62 a and 62 b which can be movedby small rams 64 a and 64 b to close off the aperture as shown inset at66. Wires 30 a and 30 b of fixed length support platform 28, beingterminated at two further strong points 68 and 70 on deck 12. Wires 30 aand 30 b pass over to additional sheaves 72 and 74 respectively whichare mounted on cross head 36 of heave compensator 34.

Fibre Rope System

FIG. 4 shows the fibre rope system in more detail, with enlarged detailof a typical joint 18 shown inset at 80. In this figure successivesections 16 a, 16 b and 16 c of man-made fibre rope 16 are labelledseparately. While these sections may in practice be 100 m or 300 m inlength, they are shown shorter in the drawings for the purpose ofillustration. The fibre rope used is far lighter, under water, than wirerope 24. If desired, and particularly for extreme depths,neutrally-buoyant rope may be specified, such that the load on the hoistsystem is effectively independent of depth.

In the condition shown, the bulk of rope 16 including joints 18 is woundon the drum of winch 20. Winch 20, serving as a rope store rather than ahoist, is driven to take up and pay out rope 16, though not to lift theweight of load 10. Further sheaves 82 and 84 are mounted respectively onheave compensator cross head 36 and deck 12 to pass the rope 16 fromdrum winch 20 over the side of the vessel, with heave compensation asrequired. The flanges of sheaves 82 and 84 are set wide enough to allowfree passage of joints 18, it being understood that the portions 16 band 16 c of fibre rope 16 are, in operation, not subject to the weightof load 10 to reduce fatigue in the fibre rope as it bends.

Referring to the inset detail 80, each joint 18 in this embodimentcomprises a symmetrical arrangement of components, permitting segmentedrope 16 to be used without regard to the direction in which it has beenwound on drum 20. At the centre of each joint 18 is a circular plate orcollar 86, of a size suitable for engagement by the latched 32 ofhang-off platform 28 (see detail 46 in FIG. 3). Rope segments 16 a and16 b either side of the joint are terminated by respective thimbles 88 aand 88 b, which are connected to either side of collar 86 by shackles 90a and 90 b. Attached rigidly to each thimble 88 a, 88 b is a respectivepart-conical stopper 92 a, 92 b. Each cone widens in the direction fromthe rope segment 16 a to the terminating eye 88 a. The lower stopper 92a is thereby adapted to be engaged by the part-conical surfaces 42 a and42 b of the hoist platform jaws 27 a and 27 b, as seen in the detailedinset at 46 in FIG. 2.

Each stopper 92 a, 92 b is formed by two halves of steel, clamped to therespective thimble 88 a, 88 b by a bolt 93. The lower stopper 92 a isshown with one half removed, to reveal the form of the thimble 88 a.This comprises a solid metal piece, with a channel guiding the rope 16along path 16′ shown in broken lines. The rope doubles back and isspliced to itself in conventional fashion, the splice extending perhaps3 or 4 m for security from the thimble. Unlike conventional thimbles,the thimbles 88 a and 88 b provide two distinct apertures in a solidbody. The larger aperture allows passage of the shackle 90 a, 90 b whichconnects, via collar 86 and other parts, to the next rope segment. Asmaller aperture 91 allows passage of the bolt 93 which, indirectly inthis embodiment, allows connection of the rope segment 16 a to the hoistplatform 22. The body can be formed entirely by casting, or assembledfrom a split tube and other pieces.

Finally in relation to FIG. 4, it can be seen that a further conestopper 94 is provided at the connection between the lowermost ropesegment 16 a and the load 10. Stopper 94 again is dimensioned andoriented to be engaged by the jaws 27 a and 27 b of hoist platform 22,as part of the lowering or lifting sequence, which will now be describedin more detail.

Set-up for Lifting and Lowering

Summarising the configuration of the apparatus just described withreference to FIGS. 1 to 4, an ordinary, cylinder-based heave compensator34 is provided, comprised of a hydraulic ram 38 with cross head 36 uponwhich sheaves are mounted, together with a winch system (26, 22) whereinthe hoist medium is wire rope (24).

The wire 24 is taken from the winch 26 and passed through sheave 58 onthe deck. It is then reeved through first sheave 54 on the heavecompensator cross head and through a lowerable block in the form ofplatform 22 and sheaves 48, 50, 52. The standing part is then returnedto the ship and reeved through the second sheave 56 on the heavecompensator and subsequently made fast to strongpoint 53 on the deck.This provides a heave compensated double-fall lowering (and lifting)system.

As described above with reference to FIG. 3, wires 30 a and 30 b are ledfrom two further strong points 68, 70 on the deck to hang-off platform28 via the two additional sheaves 72, 74. The platform 28 is thus heavecompensated at the same rate and in phase with the lowering system.

Man made fibre rope 16 (details in FIG. 4) is provided in lengths thatare the same length (or shorter) as the travel of the lowering systemhoist platform 22. These are terminated in eyes and joined together toform a continuous length suitable for the depth of the work and wound onto reel 20. The rope 16 is then reeved through the deck and cross headsheaves 84, 82.

Lowering Operation

With the complete apparatus constructed and prepared as just described,operation of the apparatus for lowering of a heavy load to the seabedproceeds as will now be described with reference to FIGS. 5 to 9. Itwill be appreciated that lifting operations can be performed by a simplereversal of the lowering process.

FIG. 5 shows the apparatus in an initial condition. Load 10 haspreviously been passed overboard by a suitable crane (not shown), andsuspended by stopper 94 in the jaws of hoist platform 22. Rope 16 hasbeen passed through the aperture 60 in hang-off platform 28, andconnected to stopper 94 by a shackle. The exact sequence of theseoperations can be varied to suit. Either platform 22 or 28 may be madeopenable to facilitate entry of the rope into the respective aperture40, 60.

By operation of wire winch 26 paying out wire 24, the platform 22 andload 10 are lowered to the maximum depth of the lowering system with therope 16 attached, thus drawing the rope 16 off the reel 20.

Reaching the position shown in FIG. 6, the first joint 18 a in the rope16 will now be at the hang-off platform 28 and a latch 32 is engagedwhich grips the collar of the joint.

Referring now to FIG. 7, the load is now transferred to the rope 16 andhang-off platform 28, while the jaws 27 a, 27 b on the travelling block(platform 22) are released and recovered to the surface (FIG. 8). Oncethe hoist platform 22 is appropriately close beneath the hang-offplatform 28, it connects with the lower stopper 92 a of joint 18 a inthe rope 16 and takes the load again. The joint's collar 86 is releasedfrom the platform 22 and lowering may once again be undertaken.

In this manner the rope is never subjected to more than nominal loadwhile being bend cycled through the winch, sheaves and especially theheave compensator. At the same time, all transfer operations are heavecompensated.

Support for the process may be provided by a remotely operated vehicle(ROV) and/or divers, not shown. These may be stationed by the lowestposition of the hoist platform 22, for example, where video observationand occasional intervention may be required to ensure reliableengagement and disengagement of the hoist from the rope stoppers 92 aetc. Adequate observation may also or alternatively be provided bycameras mounted on the platform 22 and/or 28.

The method can be applied beneficially in oil & gas field development(sub-sea construction) in depths beyond 300 m. General lifting andlowering operations can also be envisaged in depths down to full oceanicdepth, for example for Salvage, Oceanography, and Military purposes.

1. Method of lowering equipment from a vessel to the seabed and/orlifting equipment from a seabed to a vessel using man made fibre ropeand a winch employing wire rope, wherein the fibre rope is paid out or,as the case may be, drawn in, in sections by repeated operation of thewinch and wire rope.
 2. A method as claimed in claim 1 wherein the fibrerope is continuous and is provided with eyes, stoppers or otherattachment points at regular intervals.
 3. A method as claimed in claim1 wherein the fibre rope is continuous and comprises discrete sectionsterminated with eyes or other attachment points, connected together toform the required length.
 4. A method as claimed in claim 3 wherein theconnections between sections are made in the course of paying out.
 5. Amethod as claimed in claim 3 wherein the connections between sectionsare un-made in the course of drawing in.
 6. A method as claimed in claim3 wherein the entire length is connected in advance and unreeled or, asthe case may be, reeled during operation.
 7. A method as claimed inclaim 3 wherein each fibre rope is terminated by looping around athimble comprising a body with a peripheral channel for the rope, andtwo distinct openings, the first opening, receiving a connection, eitherdirectly or indirectly, to the next rope sections, the second openingreceiving a connection, either directly or indirectly, to the hoistmechanism.
 8. A method as claimed in claim 7 wherein the second openingreceives a bolt which fixes a stopper to the thimble, the stopper beingtemporarily engaged by the hoist mechanism.
 9. A method as claimed inclaim 8 wherein the stopper comprises a part-conical member formed intwo halves held together by the bolt.
 10. A method as claimed in claim 8wherein the fibre rope is provided between adjacent rope sections withstoppers engaged and released by a hoist mechanism openable by remotecontrol to engage and release the fibre rope.
 11. A method as claimed inclaim 8 wherein the wire rope is arranged to raise and lower a hoistmechanism, the hoist mechanism being operable by remote control to allowthe mechanism to pass freely up and down the rope, and then to engagethe rope by means of said stopper.
 12. A method as claimed in claim 11wherein the fibre rope is held at a holding means, while the wire ropeis detached for connection to a next section.
 13. A method as claimed inclaim 12 wherein the holding means are arranged to engage an attachmentpoint between adjacent rope sections but distinct from that engaged bythe hoist platform.
 14. A method as claimed in claim 13 wherein saidholding means are pair of collars or other stoppers, spacedlongitudinally to allow sufficient clearance between hoist mechanism andthe holding means.
 15. A method as claimed in claim 1 wherein the wirerope winch is heave compensated.
 16. A method as claimed in claim 1wherein the wire rope winch and holding means may both beheave-compensated, such that heave compensation can be maintainedthroughout the process of transferring the load from one to the other.17. A method as claimed in claim 16 wherein the holding means and wirerope winch are heave compensated in parallel by a common heavecompensator.
 18. A method as claimed in claim 17 wherein said commonheave compensator comprises a hydraulic ram.
 19. A method as arranged inany preceding claim wherein the winch and wire rope is arranged in adouble fall arrangement, with the end of the wire rope fixed on boardthe vessel while a running block is lowered and raised, alternatelyconnected to and disconnected from the fibre rope.
 20. Apparatuscomprising a rope store, hoist means and holding means adapted fordeployment from a sea-going vessel for implementing a method of raisingor lowering according to the invention as claimed in claim
 1. 21. Afibre rope assembly comprising plural rope sections and load-bearingstoppers connected between the sections, the rope assembly being adaptedfor use with a method according to the invention as claimed in claim 1.23. A thimble comprising a body with a peripheral channel for the rope,and two distinct openings, the first opening for receiving a connection,either directly or indirectly, to an elongate object, the second openingreceiving a connection, either directly or indirectly, to a hoistmechanism.
 24. A thimble as claimed in claim 23 wherein said elongateobject is a rope.